It is known that, when extruding metal using reverse method, the various layers of the extruded billet are substantially found in the extruded product, from the outside inwards. In particular, the outer layer of the billet appears, after extrusion, on the surface of the extruded products.
In point of fact, the surface of metal billets for extrusion originating from foundries normally exhibits surface defects, such as casting skin, segregation of alloy or addition elements, cold shot, etc. These defects may be found over a certain thickness which may be, for example, of the order of 2 mm starting from the outer face of the billet.
It has always been assumed that, before introducing the billet to be extruded into the container, it was necessary to remove the outer layer likely to contain the surface defects.
In a first known method, this removal is achieved via a peeling operation on a lathe. This method is effective but obviously increases production costs and requires turning installations for preparing the billets. In another method, the outer layer is removed by scalping, causing the billet to pass over blades which plane the outer surface over the desired thickness. This method is more rapid than peeling, but it must be performed in the hot state using a special device placed either between the furnace for heating the billets and the extruding press or directly on the press.
These two methods make it possible to obtain good surface quality for the billets, but present certain disadvantages. Firstly, the thickness of metal to be removed must take into account the geometric tolerances over the diameter, surface roughnesses, the thickness of the outer layer exhibiting structural defects and the precision of the preparation device, and the result of this is that a relatively large thickness, which may range up to 10 mm, has to be removed this involves a considerable percentage of waste before loading in the press. In point of fact, fire loss when resmelting the waste from turning is very considerable.